1. Technical Field of the Invention
The present invention relates to electrophoretic devices, and particularly relates to an electrophoretic device having opposing electrodes, members comprising an insulating material, and an electrophoretic dispersion liquid containing a liquid dispersion medium and electrophoretic particles, wherein the members and the dispersion liquid are placed between the opposing electrodes.
Furthermore, the present invention relates to various electronic devices including an electrophoretic display device having an electrophoretic device.
2. Description of the Related Art
Conventionally, among electrophoretic devices having a pair of electrodes between which an electrophoretic dispersion liquid containing a liquid dispersion medium and electrophoretic particles are located, the following device is known: an electrophoretic device utilizing a phenomenon wherein the distribution of the electrophoretic particles is changed by applying a voltage between a pair of the electrodes. FIG. 10(a) is a general structural illustration showing an electrophoretic display device including the electrophoretic device.
The electrophoretic display device 20 includes a first substrate 1 having an electrode 3, a second substrate 2 having a transparent electrode 4, an electrophoretic dispersion liquid 10 packed into a space between the electrode 3 and the transparent electrode 4, and a spacer 7 having a function of preventing the electrophoretic dispersion liquid 10 from leaking out. FIG. 10(a) is a sectional view showing one pixel in a display.
The electrophoretic dispersion liquid 10 contains a liquid dispersion medium 6 and electrophoretic particles 5 dispersed in the liquid dispersion medium 6. The liquid dispersion medium 6 has a color different from that of the electrophoretic particles 5.
The electrophoretic display device 20 is connected to power supplies 9a and 9b for applying voltages in opposite directions through a selector switch 8. That is, the electrode 3 is connected to one end of the power supplies 9a and 9b, and the electrode 4 is connected to the other end of the power supplies 9a and 9b through the selector switch 8. In such a configuration, the direction of a voltage applied between the electrodes 3 and 4 can be changed by switching the selector switch 8. Changing the direction of the applied voltage causes the electrophoretic particles to gather at a desired electrode, thereby performing desired display.
Referring to FIG. 10(b), when the electrophoretic particles 5 are positively charged, applying a voltage from the power supply 9a causes the electrophoretic particles 5 to gather at the side of the transparent electrode 4, which is close to the observer. In this situation, the observer sees the color of the electrophoretic particles 5. In contrast, as shown in FIG. 10(c), applying a voltage from the power supply 9b causes the electrophoretic particles 5 to gather at the side of the electrode 3, which is away from the observer. In this situation, the observer sees the color of the liquid dispersion medium 6. When the electrophoretic particles 5 are negatively charged, the particles migrate in the opposite direction to that in the above description.
As described above, when the configuration shown in FIG. 10(a) is employed, two-color display is possible depending on the direction of an applied voltage. Therefore, electrophoretic displays for displaying desired images can be obtained by applying the configuration shown in the figure to all pixels.
Furthermore, other electrophoretic devices having electrodes covered with an insulating material are known. In the electrophoretic devices, an insulating layer is disposed on a surface of at least one of the mutually opposing electrodes and the maximum voltage applied to the insulating layer is designed to be smaller than the product of the dielectric breakdown strength and the thickness of the insulating layer. In such a configuration, the electrophoretic dispersion liquid can be prevented from deteriorating due to carrier transfer between the electrophoretic dispersion and the electrodes.
It is expected that electronic paper can be achieved by providing the above electrophoretic devices on a flexible substrate. In such electronic paper, it is necessary to hold the display content for a long time after the display content is written by applying a voltage.
In an electrophoretic device including electrodes provided with an insulating film thereon, when the time constant of the insulating film is adjusted to be larger than the time constant of the electrophoretic dispersion, holding the display content for a long time is impossible due to self-deletion.
When the time constant of the insulating film is reduced, self-deletion can be prevented. However, it is difficult to apply an electrical field having intensity sufficient to cause the electrophoretic particles to migrate in the electrophoretic dispersion liquid. Furthermore, there is a risk that the insulating performance of the insulating film will degrade, deteriorating the electrophoretic dispersion.
The present invention has been developed in order to solve the above disadvantages of conventional methods, and it is a first object of the present invention to provide an electrophoretic device in which the electrophoretic dispersion liquid is prevented from deteriorating and the image-retention characteristics are greatly improved to obtain, for example, electronic paper.
Furthermore, it is a second object to provide an electronic device including a display such as electronic paper, wherein the display is an electrophoretic display device and has greatly improved image-retention properties.
In order to solve the above problems and to achieve the first object, an electrophoretic device of the present invention includes a first substrate, a first electrode disposed on the first substrate, a second substrate, a second electrode disposed on the second substrate and facing the first electrode, an electro-optical layer having an electrophoretic dispersion liquid containing at least a dispersion medium and electrophoretic particles and located between the first and second electrodes, and an insulating member disposed between the first and second electrodes, wherein a voltage applied between the first and second electrodes has different values, and the time constant of the voltage is larger than the time constant of the insulating member when the voltage changes from one of the different values to another.
According to the above configuration, when the time constant of the insulating member is larger than that of the electrophoretic dispersion liquid, self-deletion can be prevented, thereby significantly improving the image-retention characteristics.
In the electrophoretic device of the present invention, the insulating member may be disposed at least either between the first electrode and the electro-optical layer or between the second electrode and the electro-optical layer. The first substrate and the first electrode may both have optical transparency, and the insulating member may be disposed between the first electrode and the electro-optical layer and may have optical transparency.
The following relationship is preferably satisfied:
Rxc3x97Cxe2x89xa7Repxc3x97Cep
wherein R represents the electrical resistance of the insulating member, C represents the capacitance of the insulating member, Rep represents the electrical resistance of the electrophoretic dispersion liquid, and Cep represents the capacitance of the electrophoretic dispersion liquid.
According to this configuration, a high voltage can be applied to the electrophoretic dispersion liquid effectively.
The insulating member may include an insulating film disposed on at least one of the first substrate and the second substrate.
According to this configuration, the resistance and the capacitance can be adjusted by setting the type and the thickness of the insulating film so as to satisfy the above condition.
When the electro-optical layer includes a plurality of capsules containing the electrophoretic dispersion liquid, the insulating member may include the wall film of the capsules. Furthermore, when the electro-optical layer further includes a binder disposed at least either between the first electrode and the capsules or between the second electrode and the capsules, the insulating member may include the binder.
In the above configuration, since the electrophoretic particles are limited to migrate in one capsule, the distribution of the electrophoretic particles is uniform in the electro-optical layer, thereby achieving an electrophoretic device having high reliability. In this configuration, the wall film of each capsule or the binder functions as the insulating member according to the present invention.
The waveform of a voltage applied between the first electrode and the second electrode preferably changes with a time constant larger than the time constant of the insulating member after the voltage is interrupted.
According to this configuration, the waveform of the drive voltage can be freely set when the voltage is supplied, that is, the voltage waveform can be determined in order to achieve a good display response.
In an electrophoretic device according to the present invention having a first electrode, a second electrode facing the first electrode, an electro-optical layer having an electrophoretic dispersion liquid containing at least a dispersion medium and electrophoretic particles and located between the first and second electrodes, and an insulating member disposed between the first and second electrodes, a method for driving the electrophoretic device includes applying a signal voltage between the first and second electrodes, wherein the signal voltage has different values and has a smooth waveform when the signal voltage changes from one of the different values to another.
In the method for driving an electrophoretic device according to the present invention, a signal voltage having a smooth waveform is applied between the first and second electrodes when the signal voltage changes from one of the different values to another. Therefore, the application of a reverse-polarity voltage to the electrophoretic dispersion liquid can be prevented, thereby improving the image-retention characteristics.
In the method for driving an electrophoretic device according to the present invention, the time constant of the signal voltage is preferably larger than the time constant of the insulating member when the signal voltage changes from one of the different values to another. The waveform of the voltage applied between the first and second electrodes is preferably smoothed with a time constant larger than the time constant of the insulating member after the voltage is interrupted. The operation and the advantages are the same as those described above.
In an electrophoretic device according to the present invention having a first electrode, a second electrode facing the first electrode, an electro-optical layer having an electrophoretic dispersion liquid containing at least a dispersion medium and electrophoretic particles and located between the first and second electrodes, and an insulating member disposed between the first and second electrodes, a circuit for driving the electrophoretic device includes a voltage-generating circuit for generating a signal voltage having a plurality of different values, a selection circuit for selecting any one of the different values of the signal voltage generated in the voltage-generating circuit, and a waveform-shaping circuit for smoothing the waveform of a signal voltage output from the selection circuit when the signal voltage is changing.
According to the circuit for driving an electrophoretic device according to the present invention, a signal voltage having a smooth waveform can be supplied to either the first substrate 1 or the second substrate 2 when the signal voltage changes from one of the different values to another. Therefore, the application of a reverse-polarity voltage to the electrophoretic dispersion liquid can be prevented, thereby improving the image-retention characteristics.
In order to achieve the second object described above, the electronic device according to the present invention has a display for displaying data, wherein the display includes an electrophoretic display device having an electrophoretic device of the present invention.
According to such a configuration, even if the display is of the electrophoretic type, an electronic device including a display having greatly improved image-retention characteristics can be obtained.